In a turbo charger (exhaust turbo charger) used in an internal combustion engine of an automobile or the like, there is often used a radial turbine or a diagonal flow turbine configured such that a turbine rotor is rotationally driven by causing exhaust gas to flow from a spiral scroll portion formed in a turbine housing toward a rotor blade of the turbine rotor positioned inside the scroll portion in a radial direction, act on the rotor blade, and then flow out in an axial direction.
FIG. 4 illustrates Japanese Patent Application Laid-open No. 2003-120303 (Patent Document 1) showing an example of the turbo charger using the above radial turbine as a conventional art. In the drawing, 01 denotes a turbine housing, 04 denotes a spiral scroll portion formed in the turbine housing 01, 05 denotes an exhaust gas outlet passage formed in the inner periphery of the turbine housing 01, 06 denotes a compressor housing, and 09 denotes a bearing housing which connects the turbine housing 01 and the compressor housing 06.
010 denotes a turbine wheel, and a plurality of rotor blades 03 are fixed to the outer periphery of the turbine wheel at regular intervals in a circumferential direction. 07 denotes a compressor impeller, 08 denotes a diffuser provided at an air exit of the compressor impeller 07, and 012 denotes a rotor shaft which connects the turbine wheel 010 and the compressor impeller 07. 011 denotes a pair of bearings which are attached to the bearing housing 09 to support the rotor shaft 12.
L1 denotes an axial center of rotation of the turbine wheel 010, the compressor impeller, and the rotor shaft 012.
In the turbo charger with the radial turbine, exhaust gas from an internal combustion engine (not shown) enters into the scroll portion 04, flows into the rotor blade 03 from an entrance end surface on the outer peripheral side of the rotor blade 03 while spinning along the spiral shape of the scroll portion 04, flows toward the center of the turbine rotor 02 in a radial direction, performs expansion work on the turbine rotor 02, flows out in the direction of the axis L1 of the rotor shaft 012, and is sent to the outside of the turbo charger from the exhaust gas outlet passage 05.
FIG. 5(A) shows the vicinity of a tongue portion formed in the inner periphery of an exhaust gas entrance of the radial turbine of Patent Document 1, and is a schematic cross-sectional structural view in a direction orthogonal to the axis L1 of the rotor shaft 012, while FIG. 5(B) is a view obtained when viewed in a direction indicated by an arrow W of FIG. 5(A).
In FIG. 5(A), 04 denotes the scroll portion, 044 denotes an exhaust gas inlet, and 045 denotes a tongue portion. A structure is adopted in which the tongue portion 045 separates a flow path 046 as a connection portion, through which the exhaust gas from the exhaust gas inlet 044 passes to be guided to the scroll portion 04, from a rotor blade side passage 047 through which the exhaust gas flows into the rotor blade 03.
As shown in FIG. 5(B), the tongue portion 045 receives exhaust gas heat from the side of the flow path 046 and the side of the rotor blade side passage 047, while a heat radiation path of the tongue portion 045 for the heat accumulated in the tongue portion 045 is narrow as indicated by an arrow Z (see FIG. 5(A)), and heat radiation efficiency of the tongue portion 045 is poor.
In addition, the cross-sectional shape of the scroll portion 04 becomes wider in the direction of the axis L1 of the rotor shaft 012 and a shroud portion X is deep so that the exhaust gas heat tends to stay and the heat radiation efficiency at the corresponding portion is poor.
Consequently, the temperature of the tongue portion 045 sometimes reaches 800 to 900° C.
Patent Document 1: Japanese Patent Application Laid-open No. 2003-120303